The invention relates to a method for impregnating and embedding electrical windings, especially coils of transformers, by means of impregnating or casting resins of polyepoxy-polyisocyanate mixtures in the presence of a reaction accelerator which is latent under the processing conditions.
In electrotechnology, the impregnation and casting-in of windings in electric machines and in coils of dry transformers is of great technical and economical importance. For improving the power/weight ratio and for higher thermal stressability, reaction resin molded materials with good mechanical, thermal and electrical properties are required. Special requirements as to the molded materials arise if, for instance, in the case of transformers, the potential breakdown is not effected by air cooling sections, but it to be effected largely in the reaction resin molded material. Also, the reaction resin molded materials are subjected to high mechanical and thermal stresses when a temperature cycle resistance to -50.degree. C. is to be provided and at the same time, for example in the case of transformers, the requirements of temperature classes F to H are to be fulfilled.
Consequently, effective resistance against such destructive stresses in reaction resin molded materials is exhibited by a molded material which contains in its structure oxazolidinone and isocyanurate rings. Such a molded material is produced from resin ingredients of polyepoxides and polyisocyanates. These resin ingredients are abbreviated EP/IC resins herein and the corresponding molded materials are abbreviated OX/ICR molded materials herein.
It is known that EP/IC resins employ hardening, i.e., cross-linking, which generally takes place in the presence of accelerator systems (catalysts) and which results in production of OX/ICR molded material with excellent dimensional heat stability (see, for instance: DE-AS No. 1 115 922; col. 5, lines 9 to 14; DE-AS No. 1 963 900: col. 1, lines 4 to 13 and 48 to 60, and U.S. Pat. No. 4,070,416). It is furthermore known to use EP/IC resin as an impregnating resin for windings in electric equipment, especially for rotating electric machines; and transformers (see, for instance, U.S. Pat. Nos. 4,131,600; 4,239,998 and 4,304,818). Until the present invention, however, it was not known how to impregnate and embed electrical windings with EP/IC resins in a fashion which would avoid production of void spaces within the resin.
A fundamental problem with EP/IC resins is that the polyisocyanates contained therein react with water and produce CO.sub.2 gas, which creates void spaces within the resulting cross-linked OX/ICR molded material. A winding impregnated with such a molded material containing void spaces cannot be used because the void spaces permit the partial discharge of current when the winding is under electrical stress. Small amounts of water are sufficient to form disturbing CO.sub.2 gas. It can be present as an impurity of components of the EP/IC resins such as epoxy resins and fillers. Also, the small amounts of water present in the windings are sufficient to form CO.sub.2 gas.
The processing and cross-linking conditions used at present for epoxy resins are not suitable for the production of embedded windings without voids. It has been found that the conditions for drying and degassing lead neither to the complete removal of residual water contents in the components of the EP/IC resins, nor to the removal of residual water content from the windings. Complete dehydration of the windings cannot be achieved even when high drying temperatures and reduced pressure are applied. Also, expensive drying measures cannot be carried out if economical production is to be assured. Further, the technically customary procedures of hardening the impregnated and cast-in windings in annealing ovens will not produce embedments free of gas and shrinkage voids because dwelling times in the annealing oven of many hours long are needed to cause the gelling of the reaction resins. These long dwelling times cause an increased tendency to form CO.sub.2 through reaction of the isocyanate component with residual water in the winding. Moreover, the cross-linking of the EP/IC resins in the annealing oven does not yield a temperature-cycle-resistant and surge-proof embedment especially for larger wall thicknesses of the insulation, since the cross-linking of the EP/IC resins takes place from the outer zones inward. These factors lead to high internal stresses and shrinkage voids in the EP/IC resins.
Therefore, it is an object of the invention to develop procedures which are reliable and which avoid the formation of gas and shrinkage voids when impregnating and embedding electrical windings with EP/IC resins which are cross-linked to form OX/ICR molded materials.